Ditch parameter.py?

I'm not sure what parameter.py and the Parameter object now does, other than wrap around Prior.

Can we move all the functionality into Prior, this would leave Prior having attributes/methods:

I've omitted value and set_default_value as I think this is now not used anywhere and I'm not sure it's well defined for a prior to have a value.

We should change a few things, but I think these will fit more naturally into a wider discussion about specifying priors.

I think there's a more natural version of parse_floats_to_parameters, although I'm not sure what it is yet.

We will then pass a list(?) of Prior objects into the sampler for our prior, rather than Parameter objects.